This invention relates generally to programmers or monitors intended for use with implantable medical devices and more specifically to antennas adapted for communication between implanted medical devices and external programmers or monitors.
In the context of implantable medical devices, it has become common to provide a communication link between the implanted device and an external programmer or monitor in order to allow for transmission of commands from the external device to the implanted device and to allow for transmission of stored information and/or sensed physiological parameters from the implanted device to the external programmer. Conventionally, communication between an implanted device and an external programmer or monitor has been accomplished by means of a telemetry system which includes a transceiver located within the implanted medical device and an external programmer or monitor, each having a radio transmitter/receiver and an associated antenna.
The implanted device typically includes an antenna located either within the hermetic device housing containing the circuitry, as disclosed in U.S. Pat. No. 4,542,532 issued to McQuilkin, in a plastic header or connector block used to interconnect the device to electrical leads as disclosed in U.S. Pat. No. 5,697,958 issued to Patrick et al. or mounted to the device housing as in U.S. Pat. No. 5,861,019 issued to Sun et al. and U.S. Pat. No. 5,720,770 issued to Nappholz et al., all incorporated herein in their entireties. The programmer or monitor typically includes or consists of a telemetry head containing an antenna, intended to be placed on the patient""s body in close proximity to the implanted device. The telemetry head may be coupled to the external programmer or monitor by means of a cord, as disclosed in U.S. Pat. No. 5,766,232 issued to Grevious et al. In alternative systems, as described in U.S. Pat. No. 5,404,877 issued to Nolan and U.S. Pat. No. 5,113,869 issued to Nappholz, the programmer or monitor may be provided with an antenna located some feet away from the implanted device.
One common telemetry head antenna configuration is a coil antenna mounted parallel to a major surface of the telemetry head""s outer enclosure, which surface is located on or closely adjacent the patient""s body during telemetric communication. Such telemetry heads are disclosed in U.S. Pat. Nos. 5,527,348 and 5,562,714, both incorporated herein by reference in their entireties. Coil antenna telemetry systems such as these have the disadvantage that they display a signal strength minimum or xe2x80x9cnullxe2x80x9d in the region directly beneath and close to the coil. In the context of telemetry systems employing transmission antennas located at substantial distances from their associated receiving antennas, one solution to eliminating null spots has been to provide diversity antenna arrays employing antennas which are either oriented to display different polarizations or are separated spatially from one another. Examples of such antenna arrays are disclosed in U.S. Pat. No. 6,009,878 issued to Wiejand. In the context of programmers for implantable medical devices, the problem of nulls is generally addressed by the physician moving the telemetry head relative to the implanted device until an appropriate spatial relationship is attained.
The present invention is directed toward an improved telemetry head which employs two coils that may alternatively be employed to communicate with the implanted device. A first, larger diameter outer coil is generally employed in conjunction with initiation of telemetric communication between the programmer or monitor and the implanted device. The telemetry head is first placed in a location that the physician believes to be located generally over the implanted device, and the larger diameter outer coil is first employed in an attempt to initiate telemetric communication with the implanted device. In the event that successful communication is not accomplished using the outer coil, programmer or monitor attempts communication using the inner coil, under the assumption that the implanted device is located directly under some portion of the outer coil at a relatively shallow depth, falling into the null. Any resultant reduced signal strength that may be associated with use of the smaller diameter inner coil is acceptable based on the assumption that the antenna of the implanted device is located in the null associated with the outer antenna coil and thus is located in close proximity to the inner coil. It may be noted that additional coil turns may be used to implement the smaller inner coil to maintain signal strength. Only if successful communication between the programmer and the implanted device cannot be accomplished using either of the two coils, will the physician be required to move the telemetry head to an alternative location. Therefore, in the particular circumstances associated with telemetry heads used in close proximity to implanted medical devices, the multi-coil antenna array provides a simple and elegant mechanism for simplifying the process of locating the telemetry head relative to the implanted device.